1. Field of Invention
The present invention relates to a technical field of chemical and environmental protection, and more particular to a star-shaped hyperbranched polymer with a triethanolamine core, a carboxylate lateral group and a dithiocarboxylate end group, and a preparing method as well as application thereof for stabilizing heavy metals in MSWI (Municipal Solid Waste Incineration) fly ash.
2. Description of Related Arts
In recent years, “junk-besieged city” phenomenon is spreading around the world. Because waste incineration takes a small area, advantages such as waste reduction, harmlessness and recycling can be maximized. Waste incineration is becoming a generally adopted waste disposal technology in developed world countries, and also a major policy of Chinese government to deal with “junk-besieged city” phenomenon. With the increasing of “junk-besieged city” phenomenon, Chinese waste incineration scale is also increasing year by year, which had been increased from 3.699 million tons in 2003 to 35.84 million tons in 2012, sharing an increase of 969%. In 2015, waste incineration scale of China will reach 112.11 million tons (according to “twelfth five-year” national urban domestic waste treatment facilities construction plan, 2012).
However, after incineration, fly ash will be produced with an amount equivalent of 2%-5% of a junk weight. Fly ash has high leaching toxic heavy metals such as As, Cd, Cr, Hg and Pb which are listed as hazardous wastes in national standard Waste Incineration Landfill Pollution Control Standard (GB16889-2008), and cannot be directly land-filled. Fly ash must reach the control indicators of the above standard before landfill in a domestic waste landfill site.
Treatment of fly ash comprises: extraction method, heat treatment method, solidification/stabilization method, and chemical agent stabilization method. Extraction method uses water, acids, alkali, or other solvents for leaching heavy metals in fly ash. However, the heavy metals travel from a solid phase into a liquid phase, which raises a new problem of wastewater treatment, and is expensive. Heat treatment comprises: sintering, vitrification and melting, wherein the fly ash is respectively heated to 900-1000° C., 1100-1500° C., and more than 1200° C., and then cooled to form amorphous, crystalline, or homogeneous glassy products. Although, heat treatment can effectively stabilize the heavy metals, construction and operating costs are high, and energy consumption is great. Furthermore, high-temperature process will generate melted fly ash which is more toxic, comprising heavy metals with a higher concentration. The solidification/stabilization method requires chemical additives such as cement, sodium silicate and lime for stabilizing before solidified, wherein the most commonly applied method is cement solidification/stabilization due to low cost. However, weight of the original fly ash is significantly increased (nearly two times), and stabilizing ability thereof on heavy metals is insufficient. There is a possible long-term release. In case of rain, soluble salt will be bleeded; and in a long-term natural environment of acid rain, the heavy metals will be leached, resulting in soil re-contaminating.
The chemical agent stabilization method uses agents to react with toxic substances in fly ash, for converting the toxic substances to low solubility, low dissolution and low toxic substances. Compared with other stabilization techniques, chemical agent stabilization method can achieve waste harmlessness, while achieving less waste capacity or no waste capacity, so as to increase the overall efficiency and economy of hazardous waste treatment and disposal systems. The chemical agent stabilization method has been widespreadly concerned in developed countries because of low operating cost. Especially in Japan, incinerated fly ash is mainly chemical agent stabilized and land-filled.
Common chemical stabilizers are: (1) inorganic agents such as lime, phosphates, iron salts (ferrous salts and iron oxide), carbonates, sulfides (sodium thiosulfate and sodium sulfide), alumina, and sodium hydroxide; (2) organic agents such as thiourea, ethylene diamine tetraacetic acid (EDTA), salts of organic dithiocarbamate and dithiophosphate, organic phosphonates, and chitosan derivatives.
Chinese patent application 200510011651.1 discloses a method for stabilizing incinerated fly ash with soluble phosphate. Soluble phosphate comprises: phosphoric acid, sodium hydrogen phosphate, sodium dihydrogen phosphate, and trisodium phosphate, whose dosage is large during stabilizing fly ash, typically 3%-10% of the fly ash weight. Which phosphate should be used depends on pH of leached solution, wherein if pH>12, the sodium hydrogen phosphate, the sodium dihydrogen phosphate, or the trisodium phosphate is selected. However, stabilized product of PO43− is easily converted to H2PO4−, HPO42− under pH of 5-9 (Grubb D. G., Guimaraes M. S. Valencia R., 2000. Phosphate immobilization using all acidic type fly ash. J. Hazard. Mater., 76(2):217-236), resulting in re-leaching of heavy metals. If pH<5, phosphate actually losses the stabilization effect on heavy metals. Therefore, PH value is the key factor in soluble phosphate stabilization. For the fly ash, no matter treated by Chinese HVEP (Horizontal Vibration Extraction Procedure) or America TPLC (Toxicity Characteristic Leaching Procedure), the pH value of the leached solution thereof is typically no less than 10, which assures the stabilization effect of phosphate on fly ash. In the long-term natural environment, acid rain will gradually neutralize alkaline substances in fly ash, in such a manner that the fly ash is in the external environment with pH<5, resulting in increasing of heavy metal leaching speed. Therefore, anti-acid ability of the phosphate-stabilized product of the fly ash is insufficient, which increases a long-term safety risk.
Sulfide, based on insoluble metal sulfide (MS) generated, stabilizes fly ash, thereby stabilizing heavy metals, wherein a dosage is typically 3%-10% of fly ash weight (Quina M. J., Bordado J. C. M., Quinta-Ferreira R. M., 2010. Chemical stabilization of air pollution control residues from municipal solid waste incineration. Hazard. Mater. 179(1-3):382-392). However, sulfides are similar to soluble phosphate, which means that heavy metals in fly ash can only be effectively stabilized under a certain pH range.
Similarly, inorganic salts such as iron salts, carbonates and hydroxides also have problems of large dosage and poor acid resistance.
Organic stabilizers are mainly: thiourea, ethylene diamine tetraacetic acid (EDTA), organic phosphonates, organic dithio carbamate, thiophosphate, etc.
Stabilization effect of thiourea on heavy metals in fly ash is significantly better than the one of inorganic sodium sulfide. When a dosage of thiourea is 0.7% of fly ash weight, Pb Leaching amount is below the Chinese national standard limit. However, for the same results, a dosage of sodium sulfide needs to be 5% (Zhao Y. C., Song L. J., Li G. J., 2002. Chemical stabilization of MSW incinerator fly ashes. J. Hazard. Mater. 95 (1-2): 47-63). Stabilizing effect of thiourea also depends on pH value. When leachate pH<8 (leaching agent pH=1), leaching speed of heavy metals is increased (Dan Chen, Guangren Qian, Houhu Zhang, Huajun Zhu, 2006. Tea Saponin Stability and Change Solid Waste Incinerated Fly Ash Test [J]. Journal of North University (Natural Science). 27 (6): 519-523).
Stabilization effect of ethylene diamine tetra-acetic acid (EDTA) on heavy metals is also dependent on pH value, wherein the pH must be more than 10 (Yuanyuan Liu, Li'ao Wang, Xiang Lin, Zhiqiang Cui, Yu Luo, Gang Hu, 2007. Municipal Solid Waste Incineration Compatibility of heavy metals in fly ash stabilization experimental drug study [J]. Journal of Environmental Engineering, 1 (10): 94-99). Therefore, EDTA acid resistance is also poor.
Chinese patent ZL 200410067071.X discloses a technology of stabilizing heavy metals in fly ash with organic phosphonates, wherein acid resistance thereof is also poor (Zhang, Houhu et al., Organic acid HEDP stabilize heavy metals in waste incineration fly ash [J]. Environmental pollution control technology and equipment, 2006, 7 (11): 45-48).
In recent years, a new type of fly ash stabilizer appears, whose chelation group is formed by dithiocarbamate (DTC) and two dithiophosphinylidyne (DTP). For example, Chinese patent application 200410090662.9 discloses a dithiocarbamate sodium fly ash stabilizer with polyethyleneimine as a basic skeleton; Chinese patent 200710190138.2 discloses a fly ash stabilizer combining dialkyl dithiophosphate compounds with flocculants; and Chinese patent application 200810032233.4 discloses a dialkyl dithiocarbamate fly ash stabilizer. These organic stabilizers all have good stabilizing effects on heavy metals.
Evaluation of the above inorganic and organic stabilizers is based on the old Chinese national standard Hazardous Waste Identification Standard-Leaching Toxicity Identification (GB5086.2-1997). With new Chinese national standard Landfill implementation of pollution control standard (GB16889-2008), requirements for fly ash stabilization are more strict. For example, the concentration limit of leached Pb is changed from 3 mg/L to 0.25 mg/L, and the concentration limit of leached Cd is changed from 0.3 mg/L to 0.15 mg/L. Dosages of the above inorganic and organic stabilizers need to be increased for satisfying new standard, resulting in great increase of cost. For example, when treating fly ash of a waste incineration site with sodium phosphate (Zhou, Bin et al., 2009), if the dosage is 10%, the Pb leaching concentration is 0.46 mg/L, meeting old standard (GB5086.2-1997) wherein a concentration limit is 3 mg/L. However, new standard (GB16889-2008) is not satisfied. Even if the dosage is 30%, the leached Pb concentration is 0.27 mg/L, still not meeting the new standard wherein a concentration limit is 0.25 mg/L. The new standard raises higher requirements for chemical stabilizers.
Therefore, for fly ash stabilizers research, a novel, efficient, and strong acid resistance stabilizer is badly needed. Conventional stabilizer limits must be exceeded and structure must be improved for increasing combining ability with heavy metals in fly ash, so as to obtain efficient, high acid resistance fly ash stabilizers.
With the development of polymer, based on the conventional one-dimensional linear, two-dimensional cross-linked or lightly branched polymer, highly branched polymer having a three-dimensional spatial structure is developed. According to structures, highly branched polymers are divided into dendritic polymers, hyperbranched polymers, star-shaped hyperbranched polymers, and star-shaped hybrid arm hyperbranched polymers. Dendritic polymers have a regular structure. Hyperbranched polymers have an irregular structure. Star-shaped polymers are hyperbranched polymers with no less than three branches connected by chemical bonds on the same central core, wherein chemical composition of each branch is identical, and molecular weight should be no difference. Star-shaped polymers have a three-dimensional snowflake-like structure, and are a special kind of highly branched polymers. And star-shaped hybrid arm hyperbranched polymers have at least one branched chain with different chemical composition or significant molecular weight difference.
Because of the special structure of the non-linear polymer, theoretical research value and industrial potential applications thereof have aroused widespread interest. In the 21st century, the dendritic polymers have won more and more worldwide attention of scientists, and have important application prospect in fields of industry, agriculture, defense, biomedical, sustained-release materials, catalysis, etc.
According to the present invention, carboxylate and dithiocarboxylate are respectively grafted on a star-shaped hyperbranched polymer chain and an end group, so as to obtain a star-shaped hyperbranched polymer, wherein a structure thereof is novel in the world. The star-shaped hyperbranched polymers are good at stabilizing heavy metals in fly ash, and stabilized heavy metals have an excellent acid and alkali resistances, which ensures a long-term stability of fly ash in the environment.